Update implementation-notes.md and model.md

[Denz1994]

Created for #48.

It would be good to reference masses-and-springs/doc for the implementation-notes and model doc. Many of the features of masses-and-springs have been retracted, which should be listed in this implementation-notes.md.

Also, this doc should include the use of a boolean flag basicsVersion in the main model, MassesAndSpringsModel.js to differentiate the specifics for MASB. A ColorProfile has been incorporated for both Basics and non-Basics versions.

Visual elements are pretty self explanatory (masses, springs, tools, and panels are labeled).

Nothing related to the model has changed.

[Denz1994]

Notes have been updated as needed. Closing.